poster child vs kid

poster child and kid both are nouns.

Word	Noun	Adjective	Verb	Adverb
poster child	Yes	No	No	No
kid	Yes	No	Yes	No

As nouns, kid is a hypernym of poster child; that is, kid is a word with a broader meaning than poster child:

poster child: a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes
kid: a young person of either sex

Other hypernyms of poster child include child, fry, minor, nestling, nipper, shaver, small fry, tiddler, tike, tyke, youngster.

poster child (noun)	kid (noun)
a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes	young goat
	a young person of either sex
	a human offspring (son or daughter) of any age
	soft smooth leather from the hide of a young goat

poster child (verb)	kid (verb)
	be silly or tease one another
	tell false information to for fun